The invention relates to the field of analog-to-digital converters and in particular to compensating for errors in the analog-to-digital conversion process.
An analog-to-digital converters (ADC) transforms an analog input signal, preferably an electrical voltage or an electrical current, with a generally constant sampling frequency into a digital output signal (i.e., a data word), usually a digital voltage sequence or a digital current sequence.
A real-world ADC does not have an ideal transfer function (i.e., its characteristic is not ideal). Deviations lead to unwanted cross-modulation and intermodulation products. The deviations from such an ideal characteristic can be quantified by characteristic quantities. These quantities, which characterize the deviations from the ideal characteristic curve, include the differential nonlinearity (DNL) and integral nonlinearity (INL). DNL means the maximum step width error, and INL means the error between the quantized value and the ideal continuous value.
To reduce the characteristic errors, that is the errors which occur in the analog-to-digital conversion due to the deviations from the ideal characteristic curve, it is known from the prior art how to add, in the case of a known characteristic, a correction value (positive or negative) depending on the input signal or the output signal, to the digital output signal. This correction value is obtained, for example, from a table that contains the correction value corresponding (preferably) to every possible input signal or every possible group of input signals or to every output signal that can be converted from the input signal or every group of output signals.
The characteristic (i.e., the conversion or transformation function) of an ADC can be found by applying a defined and known input signal. One usually employs signal ramps (e.g., voltage ramps), triangular or sine signals (e.g., voltages with triangular or sinusoidal amplitude) to determine the conversion or transformation function of the ADC. U.S. Patent Application 2004/0233083 discloses a compensation circuit that determines the conversion or transform function using a sine signal applied at the input. At the output, the transformed output signal is compared to a reconstructed ideal sine signal. The parameters for amplitude, phase, and DC voltage component (offset) of this reconstructed sine signal are obtained from the transformed output signal. The values for the correction table are found from the differences, dependent on the input signal, between the transformed output signal and the reconstructed sine signal.
These techniques measure the characteristic of an ADC either for: (i) a quasi-static signal (e.g., in the case of a slow signal ramp), (ii) a particular signal gradient (e.g., in the case of a triangular signal) or (iii) a particular frequency (e.g., in the case of the sinusoidal signal). But depending on how the ADC operates, its characteristic is dependent on frequency or gradient. If the input signal (useful signal) includes alternating or a plurality of dominant frequency components, a compensation that relies on a single measurement frequency or a single signal gradient cannot be optimal.
Furthermore, when determining the conversion or transformation function, one often factors out ranges with very high input signal amplitudes (e.g., maximum positive input voltages or minimum negative input voltages) and replaces the correction values of these amplitude ranges in the correction table with values computed from neighboring ranges. This, as well, leads to a suboptimal error compensation.
A further drawback results from the thermal dependency of a real-world circuit. The longer an ADC is in operation, the more its characteristic differs from the one measured when the converter is switched on. A second measurement would entail an interruption in service, which in many instances is undesirable or even impossible (e.g., when the converter is part of a radio or television set).
U.S. Pat. No. 4,996,530 discloses a continuous self-calibration method and device based on statistical principles. One supplies a random noise signal to the input of a system, in particular, an ADC. The function of the output signal occurring at the output of the system is statistically analyzed by correlating the output signal with the random noise signal supplied at the input. An error correction is then performed for the output signal as the result of the correlation. This technique has the advantage over those described above that an error correction with high precision is possible, one which accounts for the current operating state (e.g., the temperature at the moment) of the converter and the associated transform function. However, this technique is comparatively expensive and generally requires many computation operations, so that it is only of limited use in ADCs.
U.S. Pat. No. 5,594,612 discloses a technique for correction of nonlinearity in an analog-to-digital conversion process. In particular, this patent advocates the use of a two-tone input signal to determine the correction values (“compensation coefficients”). Although this technique may lead to more reliable compensation of characteristic errors than the above-presented method based on a single measurement frequency, still this technique is again suboptimal for frequency or gradient-dependent converter characteristics. Furthermore, the (thermal) behavior of the converter as a function of its operating time is not taken into account.
U.S. Pat. No. 5,870,041 discloses an ADC with digital compensation in which, in an operating mode to calculate calibration values, correction values (“calibration values”) are calculated in a multi-step process and used in a normal operating mode for correction of the output signal. This technique is comparatively expensive and does not it take into account, for example, the temperature function of the ADC.
There is a need for an improved technique that compensates for characteristic errors of an ADC.